Eric Gemin and Ben Roberds, Redpath/FKCI Waneta Tunnelers

Construction of twin penstocks for the new Waneta expansion project hydropower scheme in British Columbia, Canada, presented particular challenges for placing the final in-situ concrete lining on the steep, 17% gradient, penstocks. Eric Gemin and Ben Roberds of the Redpath/Frontier-Kemper Constructors partnership Waneta Tunnelers explain the various challenges of casting the final lining in the parallel 215m long x 11m diameter penstocks and how they were overcome.

Excavation and final lining of the parallel penstocks on the Waneta hydropower expansion project in Canada was awarded in 2011 to Redpath/FKCI Waneta Tunnelers as a subcontract to the Aecon/SNC-Lavalin JV as the civil works contractor to the main design-build contract held by SNC/Lavalin. The owner of the Can$900 million expansion scheme is the Fortis BC Inc partnership of Columbia Power Corporation and the Columbia Basin Trust.

Drill+blast excavation of the large penstocks was designed initially as a modified horseshoe-shaped profile with a split sequence top-heading and full sequence bench excavated after breakthrough of the top heading. Good rock conditions, once into the excavation process, allowed blasting to proceed on a full-face top heading of 5m high x 11m wide. The penstock profile was also changed to a circular section before excavation began, due primarily to difficulty in designing a cost effective ground support system for a shape with vertical sides and a flat invert. The round section was determined to be stable, and without using steel reinforcement in the final lining. The change in shape realized savings to the overall cost when compared to the earlier profile design.

Plan of the Waneta hydro scheme penstock

Top heading and bench excavation

Excavation progressed from an access adit opened at the downstream end adjacent to construction work of the new underground powerhouse. Use of the adit allowed the penstock works to proceed concurrent with excavation and construction of other structures of the scheme and stay off the project's critical path.

Permanent support in the steep 17% grade drill+blast excavation consisted of immediate shotcrete and fully grouted 4m long x 22mm diameter rebar dowels. The support was designed so that the final 10.5m i.d. in-situ final lining would face no ground loads. Loading of the final lining would come instead from the scenario of rapid dewatering of the pressurised tunnel. A hydraulic head of up to 70m would be outside the dewatered tunnel. The lining is 35MPa plain concrete and cast to 300mm thick to resist differential hydraulic pressure. No reinforcement was used, not even to reduce shrinkage. Minor cracking is in fact preferred to help drain groundwater and reduce hydrostatic pressure building on the lining. There were also no water stops or bonding agents in the cast concrete construction joints.

Tolerance and surface finish of water tunnels is particularly important. At Waneta, the limits on construction tolerance were within 12mm of line and grade and within a 52mm of differential between height and width. On the finishing quality, honeycombs of more than 25mm wide were to be filled and bug holes, caused by trapped air bubbles, of more than 5mm diameter had to be pointed. A rubbed finish was required for areas where more than 50 such bug holes occur per square meter. While irregularities of up to 3mm high were allowed, they were restricted in size and number.

Concrete form
To keep the lining works off the project's critical path, a self-advancing concrete form, able to allow a full pour cycle every 24 hours, to operate safely on a 17% slope, and also meet the surface finish requirements, was required. Proposals were called from seven suppliers and the order was placed with Ceresola Tunnel Lining Systems of Switzerland, now part of Max BöglSchweiz AG.

Preparing the self-advancing form for the in-situ concrete lining

The Swiss firm designed and fabricated a walking beam style steel form, able to cast 7.5m long full round pours, and walk itself through the steep tunnel on its own carrier. The form was designed to eliminate the need for internal supports or spud pins. Also, with a relatively short pour length of 7.5m, the external supports were sufficient to prevent movement during concrete placement. The front of the form was braced against the tunnel walls and the rear pressed off the last pour using six large screw jacks on each end; two each in the crown and invert and one each side.

For the stop-end, a steel-framed cantilever bulkhead system was provided. However, in surface trials, it proved difficult to fit the device around other installations such as walkways and hydraulic cylinders. The individual parts were also too heavy to assemble efficiently by hand. The alternative was to use rough cut 2 x 8 timbers for the bulkhead and support it using traditional 2 x 4 wood whalers and stiff-backs. As the wood supports were not designed to withstand concrete loads in cantilever they had to be pinned and braced to the rock with 20mm dowels.

To advance, rollers were installed on the carrier beams to allow the form to slide back and forth with the carrier at rest. The rollers could also be used to move the carrier when supported by the form. Not having wheels on the ground meant the form could be walked over mildly uneven surfaces, did not require a rail system, and was stable in the sloped tunnel.

Downstream adit portal

After full assembly on the surface, the form had to be partially dismantled to be lifted and lowered into the intake excavation, reassembled, and walked down the first tunnel. Installation from the intake end was required because the 6m diameter adit at the downstream end was too small for the assembled form and powerhouse construction was already underway. Partial disassembly was required because the +100 tonne form was too heavy for any viable plan to hoist the whole form into the intake excavation.

Walking the form down the tunnel to the start point was found to be a challenge - more so than progressing up. Although designed for the slope, a large moment was inflicted on the system when walking down. When the carrier was raised and extended forward, two screw jack feet on the lower end of the form were supporting the majority of the system's weight - along with resisting the moment exerted by the cantilevered carrier. Extreme care was required to prevent damaging the screw jacks during the walk down.

Form ready for the walkdown

Concrete mix design
Several concrete mixes were submitted for approval to allow flexibility based on weather conditions and temperatures. Accelerators were permitted to keep up with the pour schedule of one 7.5m long pour in each 24 hour period.

Strike specifications included a stripping strength of 4MPa within 12 hours, as measured using a maturity meter, and the accelerator, dosed typically at 181ml/100kg of cementitious materials, could only be added no more than 30 minutes before the start of discharge. A minimum slump of 180mm was needed due to limited access behind the form to place and vibrate. A slump of 220mm was too wet and allowed segregation of the aggregate. The high slump concrete also ensured that surface rock irregularities in the steep gradient tunnels were filled.

The maturity meter was used to monitor temperature and rate of hydration of the concrete. Trial batches defined the rate of compressive strength gain to the time temperature factor (TTF) output of the meter. With a TTF reading of 350 (2MPa) the bulkhead could be safely stripped, and at 450 (4MPa, the concrete was self-supporting and the steel form could be struck and advanced.

Progress rates achieved were an average of one placement every two days on the first tunnel (25 pours in 49 work days) and one placement every 1.5 days (23 pours in 37 work days) in the second tunnel. The programme of one pour in every 24 hour period eluded the process mainly due to a change in work schedules. The anticipated schedule had three crews working eight hour shifts. The actual schedule used two crews working 10 hour shifts. This gave an immediate loss of four work hours per day. The schedule also did not allow for hot changes from night shift to day shift. No pour could start any later than the beginning of a night shift. Despite these challenges, a project best of 26 hours for a full cycle was achieved on the second tunnel.

Finished concrete lining in 10.52m i.d. penstock

Another unexpected challenge was the formation of excessive bug holes. Concrete placed below spring line in the full-round form was cast against the broad surface of the form, and trapped air bubbles could not slide along and escape, due to the large shallow curve.

A rubbed finish was used to overcome the problem. A stiff sand and cement grout was troweled over the concrete surface and allowed to take an initial set before being struck off to a flat surface. This was a laborious process. The number of man-hours spent finishing the concrete was comparable to the number spent placing it.

Efforts to improve the finished concrete surface included monitoring and fine-tuning placement procedures, testing various form release agents, and minor adjustments to the mix design. While the fine-tuning efforts resulted in more efficient placing procedures and consistent concrete mix, they had little effect on the rate of bug hole formation. The finish was made more consistent but never fully met requirements as placed.

Overall, the methodology and use of the full round form for in-situ concrete lining of the steep 17% sloping tunnel proved effective and, beyond the patching of bug holes, little remedial finish work was required. The concreting work was completed in late 2012 towards reaching the new power station's commissioning date in mid-2015.

Extract from a conference paper written by Roberds, B., and Gemin, E., of Redpath/FKCI Waneta Tunnelers and presented at the TAC (Tunnelling Association of Canada) 2012 conference, Montreal, Oct 2012, and at the RETC (Rapid Excavation and Tunneling Conference) 2013, Washington DC, June 2013.

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